“I want to remind you, this is only Phase 1 of the scandal,” he told his audience Monday night. “Phase 2, I go on Oprah next week and sob.”

David Letterman's Highs and Lows

The monologue marked one of Letterman’s most memorable, dangerous and gutsiest performances – and it was damn funny.

But how long can Letterman go on being the butt of his own jokes?

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He poked fun at his situation again Tuesday night, introducing a sullen old man as his understudy. "There's a wide variety of reasons I may not be able to continue," Letterman deadpanned.

Still, Letterman went easy on himself compared to the night before, when he bounced from comedy to apology and back – referring to his off-camera troubles in near every segment, except for a Top 10 list about raising monkeys and a Steve Martin banjo interlude.

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Self-deprecating humor long has been a potent weapon in Letterman’s comic arsenal. But jokes about his bad driving and worse hair aren’t quite in the same league as cracks about being at the center of a sex scandal.

While he’s never shied away from lampooning himself, Letterman is best known for skewering others – everyone from Clinton to Dick Cheney. We don’t watch Letterman for his self-flagellation – the puncturing of the pompous with pointed sarcasm is the main draw.

He displayed some of his old edge Tuesday, ridiculing Rush Limbaugh and verbal sparring partner Sarah Palin – suggesting that drinking one's own bodily waste would be "more fun" than reading the former Alaska governor's upcoming memoir.

Few comedians, save for the great Rodney Dangerfield, could make a career out of making fun of themselves. Letterman’s challenge is to firmly re-establish his comic persona as an irreverent outsider who roasts the characters in the news – not exactly an easy task when he's dominating the headlines.

So far, the wit that's made Letterman a late-night legend has served him well as he slogs his way through this personal and professional morass. His ratings, not surprisingly, are up since he revealed the affairs and alleged extortion plot last week, and he showed encouraging signs of returning to business as usual Tuesday night.

But the story is still unfolding, and it's unclear if or how Letterman will react publicly to potential developments. He would be wise to remember that in the long term, viewers looking for few laughs before bed could run out of patience if his life becomes a running joke.